Chemokine receptor antagonists constitute a novel class of HIV-1 inhibitors that bind to cellular proteins on the surface of CD4+ T-cells and macrophages that serve as co-receptors for HIV-1. Escape of HIV-1 from inhibition by CCR5 antagonists occurs by two mechanismsemergence of CXCR4-using viruses or emergence of viruses that remain R5 but have adapted to use CCR5 for entry despite the presence of inhibitory concentrations of CCR5 antagonists. Work completed during the current funding period has led to the characterization of vicriviroc (VCV)-resistant clinical isolates of HIV-1 subtype B and C. These viruses share a number of properties, including accumulation of mutations in the stems of the V3 loop, stimulation of replication by VCV (suggesting VCV dependence for engaging CCR5) and reduced entry kinetics compared to wild-type that are restored in the presence of VCV. Given the relatively small number of CCR5 antagonistresistant isolates studied to date, much remains to be learned about the mechanisms of escape from CCR5 antagonists. The shared features of viruses resistant to small-molecule CCR5 antagonists suggest that VCVresistant viruses can serve as an excellent model for CCR5 antagonist resistance. Specific aims for the extension period of this project are as follows: 1) To complete a detailed assessment of entry kinetics of vicriviroc-resistant HIV-1. The entry kinetics of VCV-resistant HIV-1 will be explored over a range of CCR5 and CD4 levels using Affinofile cells in the presence of various concentrations of VCV or TAK-779. 2)To determine affinity of CCR5 antagonist-resistant HIV-1 envelopes for CD4 and CCR5. The ability of monomeric gp120 or trimeric HIV-1 envelope glycoproteins resistant to the CCR5 antagonists to bind CCR5- expressing cells will be examined in collaboration with Dr. Navid Madani using established protocols. 3)To explore associations between HIV-1 co-receptor usage and host genetic factors. We will make use of an existing genome-wide SNP database and high-resolution HLA haplotyping on a subcohort of 716 treatmentnaive patients to explore the hypothesis that host genetic factors are associated with emergence of CXCR4- usino virLis.